Negative electrode for lithium sulfur battery, method of preparing same and lithium sulfur battery comprising same

ABSTRACT

Disclosed is a negative electrode for a lithium sulfur battery. The negative electrode includes a lithium metal, a pre-treatment layer, and a protection layer for the lithium metal. The pre-treatment layer has a thickness of 50 to 5000 Å and includes a lithium ion conductive material with an ionic conductivity of at least 1×10 −10  S/cm.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority of application No. 2002-63834filed in the Korean Intellectual Property Office on Oct. 18, 2002, thedisclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to a negative electrode for alithium sulfur battery, a method of preparing the same, and a lithiumsulfur battery comprising the same, and more particularly, to a negativeelectrode for a lithium sulfur battery including a pre-treatment layerwith good lithium ionic conductivity, a method of preparing the same,and a lithium sulfur battery comprising the same.

BACKGROUND OF THE INVENTION

[0003] The development of portable electronic devices has led to acorresponding increase in the demand for secondary batteries having botha lighter weight and a higher capacity. To satisfy these demands, themost promising approach is a lithium-sulfur battery with a positiveelectrode including a sulfur-based compound.

[0004] Lithium-sulfur batteries use sulfur-based compounds withsulfur-sulfur bonds as a positive active material, and a lithium metalor a carbon-based compound as a negative active material. Thecarbon-based compound is one that can reversibly intercalate ordeintercalate metal ions, such as lithium ions. Upon discharging (i.e.,electrochemical reduction), the sulfur-sulfur bonds are cleaved,resulting in a decrease in the oxidation number of sulfur (S). Uponrecharging (i.e., electrochemical oxidation), the sulfur-sulfur bondsare re-formed, resulting in an increase in the oxidation number of theS. The electrical energy is stored in the battery as chemical energyduring charging, and is converted back to electrical energy duringdischarging.

[0005] The light weight and good energy density of lithium metal hasbrought about its wide use as a negative active material for lithiumsulfur batteries. However, the good reactivity of lithium metal maycause deterioration of cycle life characteristics. Studies regarding aprevention layer have been undertaken in order to address such ashortcoming.

[0006] One of the prevention layers evaluated is LIPON (lithiumphosphorous oxy-nitride), a lithium ion conductor. The LIPON is formedby sputtering a target material such as Li₃PO₄ under a nitrogen gasatmosphere. This approach has shortcomings in that nitrogen gas and theLi₃PO₄ target material react with lithium metal to form a poorlyadhering black porous lithium composite compound byproduct on thesurface of the lithium metal.

[0007] To prevent production of the byproduct, a pre-treatment layer isdisclosed in Published U.S. patent application Ser. No. 2002/0012846 A1(USA, Moltech). The pre-treatment layer includes materials such asLi₂CO₃, derived from a reaction between gaseous material such as plasmaCO₂ and a surface of the lithium metal, or metals capable of alloyingwith lithium, such as copper.

[0008] However, the Li₂CO₃ pre-treatment layer has insufficient lithiumionic conductivity (about 1×10⁻¹² S/cm or less at room temperature), andit causes structural instability due to volume increases. In addition,this process requires additional equipment such as a plasma device, sothe production cost is high, and it requires different conditions fromthe prevention layer to complete the process.

SUMMARY OF THE INVENTION

[0009] The present invention provides a negative electrode for a lithiumsulfur battery including a pre-treatment layer with good lithium ionicconductivity and substantially no change in volume. The negativeelectrode for a lithium sulfur battery can be prepared using a simpleprocess.

[0010] In one embodiment, the invention is directed to a negativeelectrode for a lithium sulfur battery including a lithium metal; alithium ion conductive material-included pre-treatment layer formed onthe lithium metal, with a thickness of 50 to 5000 Å and ionicconductivity of at least 1×10⁻¹⁰ S/cm and a protection layer for thelithium metal formed on the pre-treatment layer.

[0011] In another embodiment, the present invention provides a method ofpreparing a negative electrode for a lithium sulfur battery. In thismethod, a pre-treatment layer is deposited on lithium metal under aninert gas atmosphere, the pre-treatment layer including a lithium ionconductive material with ionic conductivity of at least 1×10⁻¹⁰ S/cm.Thereafter, a protection layer for the lithium metal is deposited on thepre-treatment layer.

[0012] The present invention is also directed to a lithium sulfurbattery including the negative electrode, a positive electrode, and anelectrolyte. The positive electrode includes a positive active materialselected from elemental sulfur, a sulfur-based compound, and mixturesthereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] A more complete appreciation of the invention, and many of theattendant advantages thereof, will be readily apparent as the samebecomes better understood by reference to the following detaileddescription when considered in conjunction with the accompanyingdrawings, wherein:

[0014]FIG. 1 is a drawing illustrating a result obtained from an XPSanalysis of a negative electrode according to Example 2 of the presentinvention;

[0015]FIG. 2 is a drawing illustrating a negative electrode of thepresent invention; and

[0016]FIG. 3 is a drawing illustrating a lithium-sulfur battery of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0017] Additional features and advantages of the invention will be setforth in the following description, and in part will be apparent fromthe description, or they may be learned by practice of the invention.The advantages of the invention will be realized and attained by thestructure particularly pointed out in the written description and claimshereof, as well as in the appended drawings. It is to be understood thatboth the foregoing general description and the following detaileddescription are exemplary and explanatory, and are intended to providefurther explanation of the invention as claimed.

[0018] The present invention relates to a new pre-treatment layer formedbetween a protection layer and lithium metal. The pre-treatment layerprevents generation of a byproduct, a black porous lithium compositecompound with poor adhesion, that is produced by a reaction of nitrogengas and a Li₃PO₄ target material with lithium metal during the formationof a LIPON (lithium phosphorus oxy nitride) protection layer. Thepre-treatment layer is a barrier for chemical reactions, and it allowslithium ions to pass therethrough.

[0019] The pre-treatment layer includes a lithium ion conductivematerial having lithium ionic conductivity of at least 1×10⁻¹⁰ S/cm, andpreferably 1×10⁻¹⁰ S/cm to 1×10⁻⁶ S/cm. The lithium ion conductivematerial may be a compound represented by Formula 1.

Li_(x)PO_(y)

[0020] where, 2<x<4 and 3<y<5. Preferably, x is 3 and y is 4.

[0021] The compound has about 100 to 10,000 times the ionic conductivityof Li₂CO₃, which is a pre-treatment material. Such a high ionicconductivity facilitates surprisingly faster transfer of lithium ions ascompared to Li₂CO₃.

[0022] The compound has no substantial change in volume, therebypreventing the shortcomings of the conventional pre-treatment materialassociated with a change in volume.

[0023] A negative electrode according to the present invention includesa lithium metal 10, the pre-treatment layer 12 on the lithium metal 10,and a protection layer 14 for the lithium metal 10 on the pre-treatmentlayer as shown in FIG. 2.

[0024] The pre-treatment layer preferably has a thickness of 50 to 5000Å. A thickness of less than 50 Å makes it difficult to work as abarrier, and a thickness of more than 5000 Å causes an increase inresistance to lithium ionic conductance.

[0025] The protection layer for the lithium metal includesLi_(a)PO_(b)N_(c), where a is 2 to 4, b is 3 to 5, and c is 0.1 to 0.9,and it is preferably Li_(2.9)PO_(3.3)N_(0.46). The thickness of theprotection layer is 1000 Å to 50 μm. A thickness of less than 1000 Åmakes it impossible to work as a protection layer, and a thickness ofmore than 50 μm increases the thickness of the electrode, therebydecreasing energy density.

[0026] A method of preparing the negative electrode will now bedescribed in more detail.

[0027] A pre-treatment layer is deposited on a lithium metal using atarget under an inert gas atmosphere. The pre-treatment layer includes alithium ion conductive material having lithium ionic conductivity of atleast 1×10⁻¹⁰ S/cm.

[0028] The lithium ion conductive material is preferably a compoundrepresented by Formula 1, and it is stable under the nitrogen atmospherewhich uses the evaporation of a lithium metal protection layer, such asLIPON.

[0029] The inert gas is preferably selected from helium gas, neon gas,and argon gas, since they do not generate byproducts from the chemicalreaction with the lithium metal.

[0030] The target may be Li₃PO₄ or a mixture of Li₂O and P₂O₅ in anappropriate mixing ratio.

[0031] The deposition process is performed by any suitable method, suchas sputtering, electron beam evaporation, vacuum thermal evaporation,laser ablation, chemical vapor deposition, thermal evaporation, plasmachemical vapor deposition, laser chemical vapor deposition, or jet vapordeposition. It will be understood to one in the related art that thedeposition method is not limited to the above methods, and it includesany conventional procedures.

[0032] Thereafter, a lithium protection layer is deposited on thepre-treatment layer using a target. The deposition is performed by anysuitable procedure and preferably under a nitrogen gas atmosphere. Thetarget may be Li₃PO₄, the same as the source material for thepre-treatment layer.

[0033] Thus, the method of preparing the negative electrode of thepresent invention can employ any standard deposition procedure. Inaddition, the method of the present invention can use Li₃PO₄ as thesource material for the pre-treatment layer, which is the sourcematerial for the protection layer, so that it is not necessary to useadditional equipment or conditions different from the procedure for theconventional protection layer, and the preparation is economical.

[0034] An example of a lithium-sulfur battery according to the inventionis shown in FIG. 3. The lithium-sulfur battery 1 includes a positiveelectrode 3, a negative electrode 4, and a separator 2 interposedbetween the positive electrode 3 and the negative electrode 4. Thepositive electrode 3, the negative electrode 4, and the separator 2 arecontained in a battery case 5. The electrolyte is present between thepositive electrode 3 and the negative electrode 4.

[0035] The positive electrode includes a positive active materialincluding elemental sulfur (S₈), a sulfur-based compound, or a mixturethereof. The sulfur-based compound may be selected from Li₂S_(n) (n≧1),organic-sulfur compounds, and carbon-sulfur polymers ((C₂S_(x))_(n):x=2.5 to 50, n≧2).

[0036] The electrolyte of the lithium sulfur battery of the presentinvention includes an electrolytic salt and an organic solvent. Theorganic solvent may be a sole solvent or a mixed organic solvent with atleast two components. The mixed organic solvent includes at least twogroups selected from a weak polar solvent group, a strong polar solventgroup, or a lithium protection group. Some electrolytes include at leastone or more solvents selected from the same group.

[0037] The term “weak polar solvent”, as used herein, is defined as asolvent that is capable of dissolving elemental sulfur and that has adielectric coefficient of less than 15. The weak polar solvent may beselected from aryl compounds, bicyclic ethers, and acyclic carbonatecompounds. The term “strong polar solvent”, as used herein, is definedas a solvent that is capable of dissolving lithium polysulfide and thathas a dielectric coefficient of more than 15. The strong polar solventmay be selected from bicyclic carbonate compounds, sulfoxide compounds,lactone compounds, ketone compounds, ester compounds, sulfate compoundsand sulfite compounds. The term “lithium protection solvent”, as usedherein, is defined as a solvent that forms a good protective layer, i.e.a stable solid-electrolyte interface (SEI) layer, on a lithium surface,and that shows a cyclic efficiency of at least 50%. The lithiumprotection solvent may be selected from saturated ether compounds,unsaturated ether compounds, and heterocyclic compounds including N, O,and/or S.

[0038] Examples of the weak polar solvent include xylene,dimethoxyethane, 2-methyltetrahydrofuran, diethyl carbonate, dimethylcarbonate, toluene, dimethyl ether, diethyl ether, diglym, andtetraglyme.

[0039] Examples of the strong polar solvents include hexamethylphosphoric triamide, γ-butyrolactone, acetonitrile, ethylene carbonate,propylene carbonate, N-methyl pyrrolidone, 3-methyl-2-oxazolidone,dimethyl formamide, sulfolane, dimethyl acetamide, dimethyl sulfoxide,dimethyl sulfate, ethylene glycol diacetate, dimethyl sulfite, andethylene glycol sulfite.

[0040] Examples of the lithium protection solvent includetetrahydrofuran, ethylene oxide, 1,3-dioxolane, 3,5-dimethylisoxazole,2,5-dimethyl furan, furan, 2-methyl furan, 1,4-oxane, and4-methyidioxolane.

[0041] Examples of electrolyte salts, which are optional for the batteryof the invention, include lithium trifluoromethane sulfonimide, lithiumtriflate, lithium perchlorate, LiPF₆, LiBF₄, tetraalkylammonium saltssuch as tetrabutylammonium tetrafluoroborate (TBABF₄), liquid statesalts at room temperature such as imidazolium salts such as1-ethyl-3-methylimidazolium bis-(perfluoroethyl sulfonyl) imide(EMIBeti), and mixtures thereof. The concentration of the lithium saltis preferably 0.6 to 2.0M, and more preferably 0.7 to 1.6M. If theconcentration of the lithium salt is less than 0.6M, the conductivity ofthe electrolyte decreases, thereby deteriorating battery performance. Ifthe concentration of the lithium salt is more than 2.0M, the viscosityof the electrolyte increases, thereby deteriorating movement of lithiumions.

[0042] The following examples illustrate the present invention infurther detail, but it is understood that the present invention is notlimited by these examples.

EXAMPLE 1

[0043] A Li₃PO₄ pre-treatment layer was formed on lithium metal bydeposition using a Li₃PO₄ target with a diameter of 4 inches under apressure of 5 mTorr, an RF power of 300W, and an argon atmosphere, for10 minutes. Thereafter, a LIPON layer for protecting the lithium metalwas formed by deposition using the same target under a pressure of 5mTorr, an RF power of 200W, and a nitrogen atmosphere, for 2 hours, tothereby produce a negative electrode for a lithium sulfur battery.

COMPARATIVE EXAMPLE 1

[0044] A negative electrode was produced by the same procedure as inExample 1, except that deposition of the pre-treatment layer was notperformed.

[0045] The negative electrode according to Comparative Example 1 had ablackish porous lithium compound on its surface, but the negativeelectrode according to Example 1 had no blackish compound and itexhibited inherent color of the lithium metal.

[0046] In order to identify the uniformity and ability to cover of theLIPON protection layer, ethanol was added dropwise to the negativeelectrodes according to Comparative Example 1 and Example 1. In thenegative electrode according to Comparative Example 1, an extreme amountof gas was generated, which indicates that the LIPON protection layerwas not uniformly formed on the lithium metal and had poor ability tocover the lithium metal. The negative electrode according to Example 1,however, generated no gas, which indicates that the LIPON layer wasuniformly formed on the lithium metal and had good ability to cover thelithium metal by the pre-treatment layer.

EXAMPLE 2

[0047] A pre-treatment layer and a LIPON protection layer were formed ona silicon wafer under the same conditions as in Example 1. Thecomponents in the pre-treatment layer and the protection layer wereanalyzed in order to identify the presence of nitrogen. The analysis wasperformed using XPS (X-ray Photo-electron Spectroscopy), and depthprofiling was performed. The result is presented in FIG. 1. It isevident from FIG. 1 that one layer with nitrogen (LIPON layer) isclearly distinguished from another layer without nitrogen (pre-treatmentlayer).

[0048] The negative electrode of the present invention has apre-treatment layer with good ionic conductivity and no volumetricexpansion. The pre-treatment layer preparation is preferably performedunder an inert gas atmosphere, rendering no contamination of the lithiummetal and a simple process.

[0049] While the present invention has been described in detail withreference to the preferred embodiments, those skilled in the art willappreciate that various modifications and substitutions can be madethereto without departing from the spirit and scope of the presentinvention as set forth in the appended claims.

What is claimed is:
 1. A negative electrode for a lithium sulfur batterycomprising: a lithium metal; a pre-treatment layer formed on the lithiummetal, the pre-treatment layer having a thickness of 50 to 5000 Å andincluding a lithium ion conductive material with an ionic conductivityof at least 1×10⁻¹⁰ S/cm; and a protection layer for the lithium metal.2. The negative electrode of claim 1, wherein the lithium ion conductivematerial is Li_(x)PO_(y), where 2<x<4 and 3<y<5.
 3. The negativeelectrode of claim 2, wherein the lithium ion conductive material isLi₃PO₄.
 4. The negative electrode of claim 1, wherein the ionicconductivity of the lithium ion conductive material ranges from 1×10⁻¹⁰S/cm to 1×10⁻⁶ S/cm.
 5. The negative electrode of claim 1, wherein theprotection layer for the lithium metal comprises Li_(a)PO_(b)N_(c),where a is 2 to 4, b is 3 to 5, and c is 0.1 to 0.9.
 6. The negativeelectrode of claim 5, wherein the protection layer for the lithium metalcomprises Li_(2.9)PO_(3.3)N_(0.46).
 7. The negative electrode of claim1, wherein the protection layer for the lithium metal has a thickness of1000 to 50 μm.
 8. The negative electrode of claim 1, wherein theprotection layer is formed on the pre-treatment layer.
 9. A method ofpreparing a negative electrode for a lithium sulfur battery comprising:depositing a pre-treatment layer on a lithium metal under an inert gasatmosphere, the pre-treatment layer including a lithium ion conductivematerial with an ionic conductivity of at least 1×10⁻¹⁰ S/cm; anddepositing a protection layer for the lithium metal on the pre-treatmentlayer.
 10. The method of claim 9, wherein the lithium ion conductivematerial is Li_(x)PO_(y), where 2<x<4 and 3<y<5.
 11. The method of claim10, wherein the lithium ion conductive material is Li₃PO₄.
 12. Themethod of claim 9, wherein the ionic conductivity of the lithium ionconductive material ranges from 1×10⁻¹⁰ S/cm to 1×10⁻⁶ S/cm.
 13. Themethod of claim 9, wherein the inert gas is selected from the groupconsisting of helium gas, neon gas, and argon gas.
 14. The method ofclaim 9, wherein the protection layer for the lithium metal comprisesLi_(a)PO_(b)N_(c), where a is 2 to 4, b is 3 to 5, and c is 0.1 to 0.9.15. The method of claim 14, wherein the protection layer for the lithiummetal comprises Li_(2.9)PO_(3.3)N_(0.46).
 16. The method of claim 9,wherein the protection layer for the lithium metal has a thickness of1000 Å to 50 μm.
 17. A lithium sulfur battery comprising: a negativeelectrode comprising a lithium metal, a pre-treatment layer formed onthe lithium metal, having a thickness of 50 to 5000 Å and including alithium ion conductive material with an ionic conductivity of at least1×10⁻¹⁰ S/cm, and a protection layer for the lithium metal; a positiveelectrode comprising a positive active material selected from the groupconsisting of elemental sulfur, sulfur-based compounds, and mixturesthereof; and an electrolyte.
 18. The lithium sulfur battery of claim 17,wherein the lithium ion conductive material is Li_(x)PO_(y), where 2<x<4and 3<y<5.
 19. The lithium sulfur battery of claim 18, wherein thelithium ion conductive material is Li₃PO₄.
 20. The lithium sulfurbattery of claim 17, wherein the ionic conductivity of the lithium ionicconductive material ranges from 1×10⁻¹⁰ S/cm to 1×10⁻⁶ S/cm.
 21. Thelithium sulfur battery of claim 17, wherein the protection layer for thelithium metal comprises Li_(a)PO_(b)N_(c), where a is 2 to 4, b is 3 to5, and c is 0.1 to 0.9.
 22. The lithium sulfur battery of claim 21,wherein the protection layer for the lithium metal comprisesLi_(2.9)PO_(3.3)N_(0.46).
 23. The lithium sulfur battery of claim 17,wherein the protection layer for the lithium metal has a thickness of1000 Å to 50 μm.
 24. The lithium sulfur battery of claim 17, wherein theprotection layer is formed on the pre-treatment layer.